The present invention relates to fiber-optic communications networks, and more particularly, to optical amplifiers with transient control capabilities for use in optical communications networks.
In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber. Optical amplifiers are used in such networks to amplify optical signals that have been subject to attenuation over multi-kilometer fiber-optic links. A typical amplifier may include erbium-doped fiber amplifier components that are pumped with diode lasers. The erbium-doped fiber amplifier stages increase the strength of the optical signals being transmitted over the fiber-optic links.
The gain of the erbium-doped fiber amplifier stages depends on the inversion level of erbium ions in the fiber. If, for example, the inversion level of a given stage is high, the gain of the stage will be high. If the inversion level of a stage is low, the gain of the stage will be low. Unless control electronics are used to maintain a steady inversion level under various operating conditions, the gain of erbium-doped fiber amplifier stages will be subject to unacceptable transients. Gain transients in an amplifier may cause fluctuations in the power of the output signals from the amplifier. If the output signals are too weak, it may not be possible to detect the signals. If the output signals are too strong, the signals may be subject to nonlinear optical effects in the fiber.
It is an object of the present invention to provide optical amplifier systems in which gain transients are controlled.
This and other objects of the invention are accomplished in accordance with the present invention by providing optical amplifiers that use a hybrid transient control scheme. Optical taps may be used to tap the main fiber path through the amplifier before and after the gain stage. This allows the input and output power of the amplifier to be monitored. The gain stage may be provided by one or more rare-earth-doped fiber coils such as erbium-doped fiber coils. The optical power obtained from the taps may be monitored using photodetectors and analog-to-digital converters.
The coils may be pumped by laser diodes or other suitable sources of pump light. For example, the coils may be pumped using one or more laser diode sources with wavelengths of 980 nm or 1480 nm. The optical output power of such laser diodes may be controlled by controlling the amount of drive current used to power the diodes.
A controller may be used to calculate the appropriate drive current to apply to the pumps based on the measured input and output signal powers of the amplifier. The control process implemented by the controller may be based on a combination of feedback and feed-forward control techniques.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.